Process of producing hydrogen fluoride as a dry gas from clear fluosilicic acid-containing solutions



NOV- 16, 1965 l.. c. OAKLEY, JR., ETAL 3,218,124

PROCESS OF PRODUCING HYDROGEN FLUORIDE AS A DRY GAS FROM CLEARFLUOSILICIC ACID-CONTAINING SOLUTIONS Filed Sept. lO, 1962 5Sheets-Sheet 1 .H TLI Nov. 16, 1965 1 c. OAKLEY, JR.. ETAL 3,218,124

PROCESS OF PRODUCING HYDROGEN FLUORIDE AS A DRY GAS FROM CLEARFLUOSILICIC ACID-'CONTAINING SOLUTIONS Filed Sept. lO, 1962 5Sheets-Sheet 2 FFECTOF Saw-ACE 7 0 VOLI/M6 /Qqr/o Q 0/(0 i 7517/0694n/.f /OO C. fers/fno# mf: 90 MIN.

0,/4 H2504 Co/ce-NTQAr/o/v: Pao/r. 82%

SUF/qcs 75 VOLL/ME 4770 INVENTOR.

NOV- 16, 1965 L. c, OAKLEY, JR., ETAL 3,213,124

PROCESS OF PRODUCING HYDROGEN FLUORIDE AS A DRY GAS FROM CLEARFLUOSILICIC ACID-CONTAINING SOLUTIONS Filed Sept. lO, 1962 5Sheets-Sheet 3 ,Qa-5100,41. /CZuo/f/NE Z Nov. 16, 1965 L, Q OAKLEY, JR.,ETAL 3,218,124

PROCESS OF PRODUCING HYDROGEN FLUORIDE AS A DRY GAS FROM CLEARFLUOSILICIC ACID-CONTAINING SOLUTIONS Filed Sept. l0, 1962 5Sheets-Sheet 4 /ZO "C 0.08

l -4 INVENTORS NGV. 16, 1955 l.. c. OAKLEY, JR., ETAL 3,218,124

PROCESS OF PRODUCING HYDROGEN FLUORIDE AS A DRY GAS FROM CLEARFLUOSILIGIC ACID-CONTAINING SOLUTIONS Filed Sept. lO, 1962 5Sheets-Sheet 5 FF/cr OF Enne/en fue@ 30 Mw @fre/w70 77M6 H1504Cones/V779 T10/y; ,4P/290x. 7576 0.05 I I UQFACE 76 l/oLuMs @4r/o: 0./ooe PRCESS F PRDUCENG HYDROGEN FLURIDE AS A DRY GAS FROM CLEARFLUGSILliCC ACD-CNTAINHNG SOLUTIONS Llewellyn C. Oakley, Jr., andTheodore T. Houston,

Tampa, Fla., assigner-s, by mesne assignments, to Tennessee Corporation,New York, N.Y., a corporation of Delaware Filed Sept. 10, 1962, Ser. No.222,526 11 Claims. (Cl. 231-153) The present application is related tocopending applications of Theodore T. Houston and Gerald E. G. Wilkinson(Serial No. 222,527), of Gerald E. G. Wilkinson (Serial No.'222,447), ofTheodore T. Houston (Serial No. 222,443), and of Fred J. K lem (SerialNo. 222,424), all of which have been assigned to a common assignee.

The present invention relates to the process of producing hydrogenfluoride as a dry gas from clear fluosilicic acid-containing solutions,and, more particularly, to a process for the manufacture of concentratedhydrofluoric acid and/or anhydrous hydrogen uoride from liuosilicic acidand/ or from a filtered solution containing a mixture of uosilicic acidand hydroiiuoric acid with the production of hydrated silica as aby-product.

It is well known that there have been many methods for the production ofhydrouoric acid and/ or anhydrous hydrogen fluoride which have been usedor proposed. Most of these prior methods employed fluospar which wascalcined with strong sulfuric acid to release hydrogen fluoride andsilicon tetrafluoride as gases. When a relatively pure fluospar wasemployed, the problem of producing fairly pure hydrogen fluoride was notso great, but when a very low-grade iluospar containing substantialquantities of silica was employed, the problem of removing silicontetrauoride was substantial and costly. When uosilicic acid was used asa source of hydrogen uoride similar problems were involved. In casesWhere large amounts of silicon tetrauoride or fiuoslicic acid wereinvolved, none of these prior methods have proved to be commerciallypracticable because they entailed very expensive processing and/ orinvolved high losses. Although attempts were made to overcome theforegoing difficulties and other disadvantages, none, as far as We areaware, was entirely successful when carried into practice commerciallyon an industrial scale.

It has now been discovered that concentrated hydrolluoric acid and/ orsubstantially pure anhydrous hydrogen fluoride can be producedcommercially and simultaneously from lluosilicic acid on an industrialscale by converting substantially all the tiuorine in said acid intohydrogen fluoride. Concentrated hydrofluoric acid and/ or substantiallypure anhydrous hydrogen fluoride can be manufactured from a mixture ofhydrouoric acid and uosilicic acid which is produced when a verylowgrade iluospar has been calcined with strong sulfuric acid and thegases absorbed in Water. A mixture of hydrofluoric acid and uosilicicacid can also be obtained when the uorine gases from phosphate rockdeuorination have been ab sorbed in water. As phosphate rock containsfrom 3% to 4% fluorine, it is a source of iluorine, and, whendeuorinated substantially all the iluorine can be recovered by the useof the present invention. Likewise, large quantities of phosphate rockare used in the fertilizer business to produce superphosphate and wetprocess phosphoric acid. In the superphosphate process from about 25% toabout 40% of the iluorine in the phosphate rock is evolved as silicontetrafluoride which is absorbed in water to produce uosilicic acid. Thereactions taking atei:

3,Zl8,l24 PatentedA Nov. i6, 1965 Since hydrated silica is a solid andis substantially insoluble, the aforesaid reactions proceed practicallyto completion.

ln the process for manufacturing wet process phosphoric acid, from about20% to about 50% of the uorine in the phosphate rock is evolved as gasesand recovered as iluosilicic acid. Fluosilicic acid is also obtainedwhen it is desired to manufacture valuable products such as, sodiumfluoride, potassium iluoride, and calcium fluoride by calcining theuosilicates of sodium, potassium and calcium. The calcination releasessilicon tetratluoride as a gas which is absorbed in water to producefluosilicic acid.

It is an object of the present invention to provide an improved processcapable of converting substantially all of the fluorine in fluosilicicacid to hydrogen fluoride while producing hydrated silica as aby-product.

Another object of the invention is to provide an improved process ofproducing hydrogen uoride which involves the treatment of a mixture oftiuosilicic acid and hydrouoric acid and the conversion of substantiallyall of the tluorine into hydrogen fluoride.

The invention also contemplates providing an improved process for theproduction of hydrogen fluoride and the simultaneous production ofvaluable hydrated silica as a by-product which is removed by filtration,etc.

The invention further contemplates providing an improved process ofproducing hydrogen uoride by converting the uorine into hydrogenfluoride obtained from fluosilicic acid now being produced in largequantities as a by-product in the production of superphosphate.

It is likewise within the contemplation of the invention to provide animproved process for manufacturing hydrogen fluoride involving the useof strong contact process sulfuric acid to dehydrate a clear solutioncontaining uosilicic acid and to decompose the iluosilicic acid into itsdry component gases while at the same time obtaining satisfactorydilution of sulfuric acid for use in the acidulation processes for theproduction of superphosphate.

It is also the purpose of the invention to provide an improved processof manufacturing hydrogen fluoride involving the retention of phosphaticvalues contained in iluosilicic acid as impurities in the sulfuric acidtreating solution and the eventual recovery in superphosphate producedwith said sulfuric acid.

Other objects and advantages will become apparent from the followingdescription taken in conjunction with the accompanying drawings, inWhich:

FIG. l is a How sheet of the improved process carrying the presentinvention into practice;

FIG. 2 depicts a curve showing the eiect of surface-tovolume ratio ofthe reacting vessel;

FlG. 3 shows a curve illustrating the effect `of sulfuric acidconcentration on the residual iiuorine in the raw material;

FIG. 4 is a series of curves showing the effect of retention time on theresidual uorine in the raw material for several temperatures;

FlG. 5 depicts a curve showing the relation of tempera ture and residualfluorine in the raw material for several retention times.

Generally speaking, the present invention contemplates aars,

The present invention involves the discovery that when aqueousfluosilicic acid and concentrated sulfuric acid are mixed, hydrogenlluoride is evolved essentially as a dry gas and simultaneously silicontetrafluoride is likewise evolved. By conducting the reaction in theabsence of free silica, a minimum amount of SiF4 is produced.

The rates at which the gases are evolved are dependent primarily on twofactors: (l) The surface area available for the release of the gases;and (l1) the relative ahnity of the sulfuric acid treating solution forhy rogen fluoride and silicon tetratluoride gases. ln the case of thefirst of these two factors, the larger the surface area for a givenvolume of solution, the more rapid is the release of the gases, whereasin the case of the second of these factors, three variables of primaryimportance are involved. These variables are:

(1) The temperature of the sulfuric acid solution containing the gases;

(2) The terminal concentration of the sulfuric acid containing thegases;

(3) The concentration of the fluorine containing7 gases in the vaporphase in contact with the sulfuric acid solution.

lt has been found that the higher the temperature, the more rapid is theevolution of gases; the higher the concentration of the sulfuric acidsolution, the more rapid is the evolution of the gases; and the lowerthe concentration in the gas phase, the more rapid is the evolution ofthe fluorine gases.

While the complete theoretical explanation of the entire mechanism ofthe reactions on which the improved process is based has not been fullyformulated, nevertheless the improved process has been satisfactory inoperation and has produced successful results. However, certainscientific and technical facts have been discovered which assisted inthe maxing of the invention. Some of these facts have been embodied incurves which are illustrated in FIGS. 2 to 5.

1n FlG. 2 a curve is depict-ed which shows the effect of surface tovolume ratio on the residual iluorine content of a sulfuricacid-fluosilicic acid mixture. To obtain the data upon which this curveis based, an experiment was performed as follows: Four agitated vesselsof the same size and shape were placed in a constant ternperature bathmaintained at 100 C. (212 F.). Fluosilicic acid of about 8%concentration (1 part) and sulfuric acid of about 93% concentration(6.85 parts) were heated and mixed to produce a mixture having a finalconcentration of about 82% sulfuric acid. lThis mixture was quicklyadded to the vessels in the bath, filling each to a different level.After 90 minutes the material in each vessel was analyzed for iiuorine.

The surface-to-volume ratio for each vessel was determined by dividingthe exposed surface area in each vessel by the volume of liquid itcontained. For example, using vessels having a surface area of squarecentimeters, that vessel to which 100 cc. was added would have asurface-to-volumc ratio of 0.3; that to which 300 cc. was added wouldhave a surface-to-vo-lume ratio of 0.1; etc. The residual fluorinecontent of the sulfuric acid solution is plotted in percent as ordinateand the surface-to-volurne ratio as abscissa. rThis curve illustratesthe importance of exposed surface in facilitating the liberation of theiluorinecontaining gases from the sulfuric acid-fluosilicic acidmixture. For example, for the conditions under which the test was made,the residual fluorine content of the mixture was about 0.15% when t esurface-to-volume ratio was about 0.08, while it was only 0.01% when theratio was 0.3.

A second group of tests was performed to obtain the data presented inFiG. 3. 1n this case, the same vessel was filled to the same level foreach test, thus keeping the surface-to-volume ratio constant.Temperature was maintained at 100 C. (212 F) Three different mixtures ofsulfuric acid and fluosilicic acid were prepared varying the ratio ofiiuosilicic acid to sulfuric acid so as to yield three differentterminal sulfuric acid concentrations between 71% and 83% sulfuric acid.in each case, the mixture was sampled and analyzed for fluorine afterthe passage of 30, 60, and minutes. FIG. 3 is a series of curvesresulting from plotting the residual iiuorine content of the solutionsexpressed in percent as ordinate against terminal sulfuric acidconcentration expressed m percent as abscissa.

The ligure illustrates the effect of increasing the terminal sulfuricacid concentration over the range from about 71% to about 83% on theresidual fluorine content of the mixed acid. it is apparent that a 10%increase in the terminal sulfuric acid content over the rangeinvestigated reduces the residual iluorine content lin said terminalsulfuric acid by a factor of about 50% at all retention timesinvestigated.

A third group `of tests was made to obtain the data utilized inpreparing FIGURES 4 and 5. A mixture of fluosilicic acid and sulfuricacid was prepared to yield a terminal concentration at approximately 78%sulfuric acid. The mixture was immediately transferred to three vesselsin such quantities that the surface-to-volume ratio for each vessel wasmade constant at 0.1. One vessel was in a C. (212 F.) constanttemperature bath, one in a C. (248D F.) bath, and one in a 140 C. (284F.) bath. Each vessel was sampled and analyzed for fluorine after thepassage of 30, 60, and 90 minutes. From these tests, data were obtainedfor construction of FGURE 4 and FIGURE 5. ln FIGURE 4 residual fluorinein the sulfuric acid expressed in percent is plotted as ordinate againstretention time in minutes as abscissa. A series of three curves wereplotted, one for each ternperature. 1n FlGURE 5 the same ordinate(residual fluorine in percent) as in FlGURE 4 is used. However, in thiscase, temperature in degrees centiprade was selected as abscissa. inthis case, a series of three curves was plotted, one for each retentiontime.

FIGURE 4 illustrates the manner in which the residual iluorine contentof the sulfuric acid solution decreases with time. At 100 C. (212 F.),it decreases from about 0.17% (30 minutes after mixing) to about 0.05%(90 minutes after mixing). At 120 C. (248 F), the decrease over the sametime period is from about 0.12% to about 0.02%, and at C. (284 F.) thedecrease is from about 0.09% to about 0.005% for the time interval.

FlGURlE 5 illustrates the manner in which the residual iiuorine contentof the sulfuric acid solution decreases with increases in thetemperature of the solution. lt was found that the residual uorinecontent after the passage of 30 minutes from the time of mixing wasabout 0.17% when the temperature was maintained at 100 C. (212 F.), andabout 0.10% when it was maintained at 140 C. (284 F). After one hour,the values were about 0.09% and about 0.01%, respectively, and after 90minutes about 0.05% and about 0.005%.

From the foregoing description, it is obvious to one skilled in the artthat an almost limitless combination of operating conditions as totemperature, reaction time, tera. minal sulfuric acid concentration, andsurface area available for evolution of gases can be employed to carrythe invention into practice. Certain practical conditions, however,require preferred operating ranges. Concentrations of iiucsilicic acidand sulfuric acid will be determined by the raw materials available.Concentration Qi the terminal sulfuric acid must not be lower than thatrequired of the subsequent use. Temperature and retention time areinterrelated in such a manner that the cost of smaller equipment for theliberation of the fluorine-containing gases at higher temperatures mustbe balanced against the cost of heating the raw materials to thesetemperatures. Surface area is limited by the type of packing used in theequipment in which the ilumine-containing gases are liberated. Ingeneral, the preferred operating ranges of the present invention are asfollows:

(l) Terminal sulfuric acid concentration is between about 65% H2804 andabout 98%H2SO4.

(2) Temperature is between about 200 F. (93 C.) and about 400 F. (204C.).

(3) Retention time has as its lower limit the ooding rate of the columnused whereas the practical retention time will depend on the conditionsof terminal sulfuric acid concentration and the temperature selected.

(4) Surface-to-volume ratio is that provided by conventional packing.

By considering the examples which are set forth hereinafter in thespecification, one skilled in the art will be able to adapt the presentinvention to the particular rev quirements of the situation.

In carrying the invention into practice, it is preferred to utilize theoperations and the equipment illustrated in FIG. l.

A supply of concentrated sulfuric acid is provided in tank A and asupply of a clear or filtered aqueous solution containing fluosilicicacid is provided in tank C. Such a solution is substantially free fromor devoid of silica. The concentrated sulfuric acid ows through line L-ato meter I where the amount of acid is measured and is controlled. Fromthe meter, the measured acid goes to heater T where it is heated to aselected and controlled temperature. Materials of construction in thispart of the equipment can be those conventionally used in the art tohandle the strength of sulfuric acid employed. The heated acid flows toreactor D for the treatment of uosilicic acid fed through line L-c andmeter I which are lined with rubber or plastic.

Reactor D is a graphite or liuorocarbon lined vessel and containsaqueous iluosilicic acid which is dehydrated by the concentratedsulfuric acid. In the treatment, the sulfuric acid is diluted by thewater of the lluosilicic acid and leaves through line L-e to tank E.From this tank, it can be concentrated for re-use in the process and isthen recycled to tank A. Instead of being recycled, the diluted sulfuricacid can be utilized in other processes, such as the treatment ofphosphate rock to make superphosphate. When the acid is so used, thedilution must be appropriate to the use.

Hydrogen fluoride and silicon tetraiuoride gases formed simultaneouslyby the decomposition of fiuosilicic acid leave the reactor via duct D-dand go to gas separator E I-n the gas separator E the substantially drymixture of hydrogen fluoride and silicon tetralluoride can be separatedin any suitable manner Well known to those skilled in the art. Forinstance, such operations include but are not limited to distillation,adsorption of hydrogen fluoride in cold sulfuric acid, or adsorption ofhydrogen fluoride in cold fluosulfonic acid. In the event that the gasseparation utilized yields a by-product of impure hydrogen fluoride,such a by-product may be recycled via line L-k to the tank C Where it isincorporated in aqueous iluosilicic acid.

Anhydrous hydrogen fluoride leaves the gas separator F via line L-l tostorage or utilization. Silicon tetrafluoride gas leaves gas separator Fand goes via duct D-f to an absorber G for silicon tetrafluoride.

Aqueous uosilicic acid is supplied from plastic or rubber lined tank B.The fiuosilicic acid thus supplied flows to meter K via line L-b andthen to absorber G, all of which have plastic or rubber linedconstruction. When the supply of lluosilicic acid is too concentratedfor eflicient absorption of silicon tetrafluoride, additional water maybe added as optionally desired to absorber G via line L-m. In theabsorber, silicon tetrauoride reacts with water under suitableconditions to form uosilicic acid and silica. The slurry of silica andfluosilicic acid flows via line If-g to rubber covered filter H wheresilica is removed by filtration or by other appropriate operation andwashed with water supplied by line L-i. The clear and ltered fluosilicicacid flows via line L-z and is recycled to tank C. Silica, which ishydrated, is removed via conveyor L-j for use as a by-product or forfurther purifying operations, etc. An alternative method of operation isto introduce the filtered fluosilicic acid to the process in tank C anduse water only through line L-m to the silicon tetralluoride absorber G.In this case, a pure fluosilicic acid is produced and may be withdrawnfrom line L-h for sale or utilization.

It will thus be observed that when the improved process embodying thepresent invention is carried into practice, strong sulfuric acid andclear, aqueous iiuosilicic acid are mixed together in a reactor. Theretention time, terminal concentration of the sulfuric acid,temperature, and surface area to volume ratio are so controlled as todehydrate iluosilicic acid and then to decompose the thus-dehydratedlluosilicic acid into essentially dry hydrogen iiuoride and silicontetralluoride gases. Such gases are evolved from the sulfuric acidsolution in the reactor.

For the purpose of giving those skilled in the art a betterunderstanding of the invention and/ or a better appreciation of theadvantages of the invention, the following illustrative examples aregiven:

EXAMPLE I Example I illustrates how the invention may be applied to theconversion of weak fluosilicic acid, produced as a by-product in themanufacture of superphosphate, into substantially dry hydrogen fluorideand silicon tetrailuoride gases. At the same time concentrated sulfuricacid as produced by the contact process is diluted to a more suitableconcentration for superphosphate manufacture.

Equipment used in this example includes a stirred reaction vesselsuitably equipped to maintain a constant temperature and an apparatusfor separation of the substantially dry hydrogen iluoride and silicontetrafluoride as well as interconnecting duct work and the like.

To the reaction vessel was added about 500 grams of 10% fluosilicic acid(H2SiF5-SiF4) and about 1715 grams of 98.5% sulfuric acid. The reactionis conducted in the absence of silica. The shape of the vessel was suchthat the addition of these materials resulted in a surface-tovolumeratio of about 0.1. The temperature of the vessel was maintained atabout 140 C. (284 F.) for about 90 minutes and all of the liberatedgases were conducted to the separation apparatus. The gas was separatedand the amount of each component measured. The quantity of silicontetraliuoride (SiF4) recovered was about 42 grams while the quantity ofhydrogen fluoride (HF) was about 7.9 grams. The residual sulfuric acidhad a terminal concentration of about HESO, and contained about 0.005%of fluorine. The SiF4 was absorbed in an aqueous solution of fluosilicicacid. In the solution, SiFd, reacted with water to produce moreiiuosilicic acid and silica. Such silica is removed as a by-product byfiltration, etc. The clear, filtered uosilicic acid is recycled forfurther treatment with concentrated sulfuric acid for the production ofhydrogen fluoride as the main product.

EXAMPLE II Example II illustrates how the invention may be applied tothe conversion of weak iluosilicic acid, produced as a by-product in themanufacture of superphosphate, into substantially dry hydrogen fluorideand silicon tetraliuoride gases. At the same time, concentrated sulfuricacid as produced by the well-known contact process is diluted to a moresuitable concentration for superphosphate manserena d ufacture. rEheoperations were conducted in a manner similar to Example I.

Equipment used in this example included a stirred reaction vesselsuitably equipped to maintain a constant temperature and an apparatusfor separation of the substantially dr/ hydrogen tiuoride and silicontetrailuoride as well as interconnecting duct work and the like.

To the reaction vessel were added about 500 grams of iiuosilicic acid(HSiFG-SiFi) substantially devoid of silica and about 1715 grams of98.5% sulfuric acid. The shape of the vessel was such that the additionof these materials resulted in a surface-to-volume ratio ot about 0.1.The temperature of the vessel was maintained at about 140 C. (284 F.)for about 60 minutes and all of the liberated gases were conducted tothe separation apparatus. rthe gas was separated and the amount of eachcomponent measured. The quantity of silicon tetrailuoride (Sili)recovered was 42 grams while the quantity of hydrogen iluoride (HF) wasabout 7.8 grams. The

residual sulfuric acid had a terminal concentration of about 80% H280.;and contained about 0.012% of tinorine.

EXAMPLE Hl Example ill also illustrates how the invention may be appliedto the conversion or" weak fluosilicic acid, produced as a by-product inthe manufacture of superphosphate, into substantially dry hydrogeniluoride and silicon tetratluoride gases. At the same time, concentratedsulfuric acid as produced by the conventional contact process is dilutedto a more suitable concentration for superphosphate manufacture. Theoperations were conducted in a manner similar to Example I.

Equipment used in this example includes a stirred reaction vesselsuitably equipped to maintain a constant temperature and an apparatusfor separation of the substantialiy dry hydrogen tluoride and silicontetrailuoride as well as interconnecting duct work and the like.

To the reaction vessel were added about 500 grams ot 10% uosilicic acid(HZSiFG-SiF4) substantially free of silica and about 1715 grams of 98.5%sulfuric acid. The shape of the vessel was such that the addition ofthese materials -resulted in a surface-to-volume ratio of about 0.1. Thetemperature of the vessel was maintained at about 120 C. (248 F.) forabout 90 minutes and all of the liberated gases were conducted to theseparation apparatus. The gas was separated and the amount of eachcomponent measured. The quantity of silicon tetraliuoride (Sil)recovered was about 42 grams while the quantity of hydrogen fluoride(HF) was about 7.6 grams. The residual sulfuric acid had a terminalconcentration of about 80% HZSO `and contained about 0.017% of iluorine.

EXAMPLE 1V Example 1V illustrates how the invention may be applied toconvert the impure weak iiuosilicic acid produced as a by-product in themanufacture of wet process phosphoric lacid into puried tluosilicic acidand hydrogen .duo-ride.

The equipment is identical to that of the previous examples except thatthe silicon tetrailuoride vapors were bubbled through an absorptiontower. rEhe slurry obtained therein was filtered and the washingscombined with the iluosilicic acid. The operations were Conducted in amanner similar to Example I.

To the reaction vessel were added about 500 grams of iluosilicie acid(HZSiFG) and about 1617 grams of 98.5% H2504. The reaction was conductedin the absence of silica and was effective to evolve vapors of hydrogeniluoride and the simultaneous Ievolution of vapors of silicontetrailuoride. The vapors evolved passed through the gas separatorcollecting the hydrogen liuoride and introducing the silicontetrafluoride into an absorption tower containing about 150 grams ofwater. The reaction vessel was maintained at about 140 C. (284 F.)

o to for about 90 minutes. The residual sulfuric acid at this timeanalyzed about 78% H2204 and contained about 0.005% of tluorine. This`acid was suitable for production of wet process phosphoric acid. Thehydrogen uoride collected from the separator weighed about 20.7 grams. The slurry collected in the absorption tower was filtered and the cakewashed with about 5 grams of water which was combined with the ltrate.The combined tiltrate and washings constituted a clar, ltered solutionof fluosilicic acid which weighed about 194 grams and analyzed 21.5%HZSiFG. The cake was oven dried and weighed about 5 grams.

EXAMPLE V Example V will illustrate a method of employing the inventionto convert substantially all of the luorine values of tluosilicic acidto hydrogen tiuoride. In this example, a stirred vessel was used as thereactor with a surface volurne ratio of about 0.1. The temperature oftreatment was controlled at about 140 C. (284 E). A retention time ofabout 90 minutes was required to evolve substantially all the fluorinein the luosilicie acid. A suitable quantity of sulfuric acid, such asabout 4430 pounds of 98.5% H280@ was caused to ilow from supply tank Avia line L-a through meter l and heater T to reactor il A supply ofiiltered aqueous fluosilicic acid, such as about 1586 pounds of 25%HZSiFS containing about 5 pounds of HF, was iiowed from supply tank Cthrough line L-c to `and through meter J and then to reactor DPreheating of the sulfuric acid in heater T and heat of dilution in thereactor D raises the temperature of the solution to about 140 C. (284F). The terminal concentration of the sulfuric acid in reactor D isdiluted to about B. (about 77.67% H2804) by water in the aqueousfluosilicic acid. By controlling the temperature to about 140 C. (2Q413.), the terminal concentration to about 77.67% sulfuric acid7 and thesurface-tovolume ratio to about 0.10 and a retention time of aboutminutes, dehydration and decomposition of the fluosilicic acid iseffected and evolution of the Vhydrogen uoride and silicon tetrauoridegases results from such dehydration and decomposition.

The diluted sulfuric acid is discharged from reactor D and goes tostorage tank Ef Such dilute sulfuric acid has a strength of about 60 B.and can be concentrated for re-use in the process and can be recycled totank Af Alternatively, the dilute acid can be diverted for acidulationpurposes in the making of superphosphate from phosphate rock as it hasthe proper dilution and concentration. The fluorine gases comprisingabout 115.3 pounds of lhydrogen fluoride, about 286.7 pounds of silicontetratluoride, and about 1 pound ot water vapor are exited from reactorD through duct D-d into gas separator Ff This separator can be any ofthe well-known types and can operate in a manner familiar to thoseskilled in the art to separate hydrogen uoride and silicontetrafluoride.

From the gas separator E about pounds of anhydrous hydrogen lluorideleaves via line l5-Z to storage or for direct utilization. About 286pounds of silicon tetraiiuoride are produced along with about 10 poundsof hydrogen iluoride and leave the gas separator F via duct D-f to thesilicon tctrafluoride absorber Gf ln this absorber, silicontetrafluoride is absorbed by about 625 pounds of 20% fluosilicie acid(HZSiFG). This acid cornes from iluosilicic acid supply in tank B vialine l5-b through meter l About 713 pounds of water is added throughinlet Lem to the absorbing tower G.

From the absorber G, the slurry, which comprises about 1584 pounds ofabout 25.3% HZSFG and about 50 pounds of silica, is discharged by lineL-g onto lter H where the silica is removed and then washed with about50 pounds of water. The silica with about 45 pounds of water and about 5pounds of H2Sil-`6 is discharged from filter H via line L-j. From ilterH,

EXAMPLE VI Using an open platinum beaker 31/2" diameter equipped with aplastic agitator, 100 ml. of 98% H2S04 was added to 100 ml. of 30%H2SiF6. This mixture would have a tiuorine content of 230 g./liter, ifthere was no evolution of fluorine. The mixture was sampled as soon asaddition of the sulfuric acid was complete. It was then held withagitation at room temperature (about 25 C., 77 F.) and samples after thepassage of 15, 30, 45, and 60 minutes.

The quantity in the beaker was determined and the samples were analyzed.The results are set forth in the following table:

Table Fluor-ine Fluorine Time, Minutes Content, Evolved,

g./l. Percent The terminal sulfuric acid concentration in this test wasabout 52% H2804. It is apparent that neither the terminal sulfuric acidconcentration nor the temperature was sufficiently high to givepractical results.

EXAMPLE VII The foregoing test was repeated and only the beaker wasmaintained at 60 C. (140 F.). The results are set forth in the followingtable:

Table Fluorine Fluorine Time, Minutes Content, Evolved,

g./l. Percent It is apparent that conditions were inadequate forpractical liberation of the fluorine.

EXAMPLE VIII The test was repeated again. In this test, the beaker wasmaintained at 90 C. (194 F.). The results are set forth in the followingtable:

Table Fluorine Fluorine Time, Minutes Content, Evolved,

g./l. Percent It is to be noted that in this test the terminal sulfuricacid concentration was 61% H2504. One skilled in the art can understandfrom the foregoing results that satisfactory conditions were almostachieved. Slightly higher temperature and/or slightly higher terminalsulfuric acid concentration and/ or slightly longer retention time wouldhave been suicient for substantially complete liberation of the uorinefor practical purposes. Those skilled in the art will appreciate thatthe proper selection of temperature, terminal sulfuric acidconcentration and/ or retention time in accordance with the presentinvention as set forth more fully hereinbefore will produce practicalresults.

Although the .present .invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the inven` tion, as those skilled in the art will readilyunderstand. For instance, the novel process is operable even though somefree silica may be present. In general, the presence of minor and/ orsmall amounts of silica can be tolerated. Such modifications andvariations are considered to be within the purview yand scope of theinvention and appended claims.

We claim:

1. In the process of 4producing hydrogen fluoride as a dry vapor fromclear fluosilicic acid-containing solutions, the improvement whichcomprises subjecting a clear uosilicic acid-containing solution to theaction of heated concentrated sulfuric acid in a closed reactor in theabsence of free silica, continuing said action until a substantialportion of hydrogen fluoride is evolved from said solution as a vaportogether with silicon tetrafluoride vapor, withdrawing said vaporscontaining said hydrogen fluoride and silicon tetraiiuoride from saidclosed reactor, separating hydrogen fluoride in said vapors from silicontetrauoride whereby hydrogen fluoride is produced as a substantially dryvapor, `reacting silicon tetrafluoride with water to produce uosilicicacid and silica, removing silica and liberating clear iluosilicic acidsubstantially free from silica, and recycling clear fluosilicic acidsubstantially free from silica to the rst operation.

2. The improved process set forth `in claim 1 in which the strength ofthe concentrated sulfuric acid is controlled sufficiently high tomaintain a high terminal concentration of sulfuric acid to effect arapid evolution of vapors containing hydrogen fluoride.

3. The improved process set forth in claim 1 in which the concentrationof the concentrated sulfuric acid in the treated solution is controlledsufficiently high to assure a terminal concentration between about 65%H2804 and about 98% H2804 thereby effectively causing the rapidevolution of vapors.

4. The improved process set forth in claim 1 in which the temperature ofthe solution is controlled sufficiently high to effect a rapid evolutionof vapors containing hydrogen fluoride.

5. The improved process set forth in claim 1 in which the temperature ofthe solution is controlled between about 200 F. (93 C.) and about 400 F.(204 C.) to assist in causing the rapid evolution of vapors containinghydrogen uoride.

6. The improved process set forth in claim 1 in which the separatedsilicon tetrafluoride is reacted with water to produce iiuosilicic acidand silica and the silica is removed to produce clear uosilicic acidwhich can be used in the first operation of the improved process.

7. The improved process set forth in claim 1 in which the separatedsilicon tetrauoride is treated with water to form a solution containingiluosilicic acid and precipitated lhydrated silica and the silica isremoved to provide a clear solution of tluosilicic acid substantiallydevoid of free silica which can be recycled to the first operation inthe improved process.

8. The improved process set forth in claim 1 in which thesurface-to-volume ratio between the surface of said reactor and thevolume of said solution is adjusted to be i 'i effective to cause therapid evolution of said vapors containing hydrogen fluoride.

9. The improved process set forth in claim l in which the action betweenthe solution and the concentrated suluric acid is continued for a periodof time suciently longrto cause the effective evolution of vaporscontaining hydrogen uoride.

lt). The improved process set forth in claim El in which the actionbetween the solution and the concentrated sulfurie acid is continued fora period of time Ysuciently long and at least about 90 minutes to causethe effective evolution of vapors containing hydrogen iluoride.

ill. The improved process set orthfin claim l in which the relativelylarge body of solution containing concentrated sulfuric acid ismaintained in the reactor and a stream of aqueous solution containinguosiiicic acid sub- References Cited by the Examiner UNITED STATESPATENTS 465,607 12/1891 Beylikgy 23-153 1,297,464 3/1919 Hechenbleikner23-153 1,367,993 2/1921 Stahl 23-153 X 1,938,533 12,/1933 Peneld 23-1532,833,628 5/1958 Molstad 23-205 3,024,086 3/1962 Cines 23-153 X MAURICEA. BRINDISI, Primary Examiner.

1. IN THE PROCESS OF PRODUCING HYDROGEN FLUORIDE AS A DRY VAPOR FROMCLEAR FLUOSILICIC ACID-CONTAINING SOLUTIONS; THE IMPROVEMTN WHICHCOMPRISES SUBJECTING A CLEAR FLUOSILICIC ACID-CONTAINING SOLUTION TO THEACTION OF HEATED CONCENTRATED SULFURIC ACID IN A CLOSED REACTOR IN THEABSENCE OF FREE SILICA, CONTINUING SAID ACTION UNTIL A SUBSTANTIALPORTION OF HYDROGEN FLUORIDE IS EVOLVED FROM SAID SOLUTION AS A VAPORTOGETHER WITH SILICON TETRAFLUORIDE VAPOR, WITHDRAWING SAID VAPORSCONTAINING SAID HYDROGEN FLUORIDE AND SILICON TETRAFLUORIDE FROM SAIDCLOSED REACTOR, SEPARATING HYDROGEN FLUORIDE IN SAID VAPORS FROM SILICONTETRAFLUORIDE WHEREBY HYDROGEN FLUORIDE IS PRODUCED ASN A SUBSTANTIALLYDRY VAPOR, REACTING SILICON TETRAFLUORIDE WITH WATER TO PRODUCEFLUOSILICIC ACID AND SILICA, REMOVING SILICA AND LIBERATING CLEARFLUOSILICIC ACID SUBSTANTIALLY FREE FROM SILICA, AND RECYCLING CLEARFLUOSILICIC ACID SUBSTANTIALLY FREE FROM SILICA TO THE FIRST OPERATION.